Welcome to Flyer Talk birder

The chances of u/g clearing are very low. I've had very mixed success on u/gs clearing even as NZ*E, even using status u/g vouchers which have higher priority than airbucks standby u/gs.

Officially u/g requests can't be cancelled once they have cleared. In practice, maybe.

Perhaps the best thing is to have a look online at your seating the day of departure - if it has already cleared at that point then select a seat you like. Failing that, check in early. If your upgrade has cleared at that point you may be able to avoid a middle seat (assuming there are other seats available for selection).

If your upgrade clears at the gate you'll almost certainly have no choice.

PS the u/g cost for economy to premium economy is 220 airbucks per leg (ie 440 for AKL-LHR).